Influenza virus-augmentation of tumor transplantation antigen (TTA) activity, that is, the increased TTA activity of membrane extracts from tumor cells infected with influenza virus, was observed in a low antigenic tumor system. Priming with influenza virus additionally enhanced the tumor protection conferred by influenza virus-augmented TTA. Crude membranes of virus-augmented TTA used as an immunotherapeutic agent suppressed tumor growth markedly in combination with surgical excision of the primary tumor and chemotherapy in mice. Complement-dependent cytotoxicity tests revealed that the content of histocompatibility antigen on tumor cell was decreased by infecting them with influenza virus or vesicular stomatitis virus. The previous inoculation of BALB/c mice with Complete Freund Adjuvant (CFA) enhanced tumor growth unexpectedly.